In general, a gas-solid fluidized bed (hereinafter, called a ‘fluidized bed’) system has a structure that the inner space of the fluidized bed system is divided into an upper space and a lower space by a gas distributor 1 mounted therein to inject gas into the lower space and the gas injected through the gas distributor 1 by gas injection means or gas injector 3 is introduced into a plenum 2 and dispersed evenly so that particulate materials flow in the upper space. The lower space of the gas distributor 1 is called a ‘plenum’ or a ‘wind box’.
FIG. 1 is a configurative diagram of a conventional fluidized bed system for showing a differential pressure measurement method in a fluidized bed. As shown in FIG. 1, in order to measure flow characteristics of gas and solid inside the fluidized bed 10, a difference between pressure of the lower space of the fluidized bed 10 and atmospheric pressure is measured.
That is, temperature is measured in real time by a temperature sensor 4 immersed inside the fluidized bed 10, and a lower end of a pressure probe 5 is located at a lower end of the inside of the fluidized bed 10 to measure a difference between pressure of the lower space of the fluidized bed 10 and atmospheric pressure.
In such a system, a fluidized state is formed when fluidization gas of an appropriate amount is evenly injected to a container, which is filled with solid, by the gas distributor 1.
FIG. 2 is a graph showing changes in fluidization region and pressure drop (differential pressure) of the solid bed according to velocity of fluidization gas. As shown in FIG. 2, if the flow velocity is low, gas simply flows spaces among solid particles and there is little movement of solid due to the flow of the gas.
A particle layer of such a state is called a ‘fixed bed’. In this instance, as gas speed is increased, pressure drop at the solid bed is also increased until it becomes equal to the weight of the solid bed. If the pressure drop becomes equal to the weight of the solid bed, drag force applied to solid by gas becomes equal to gravity, and particles start to be shaken and move from each other.
Such a state is called the minimum fluidization condition, and in this instance, superficial gas velocity is called the minimum fluidization velocity (Umf).
After that, when the gas velocity increases continuously, the pressure drop remains nearly steady, but the solid bed is expanded, the particles are separated from each other to move and the bed generally starts to have characteristics of liquid. Excess gas passes the bed in the form of a large void. The large void is called bubble similarly to a gas-liquid system, the gas velocity that a bubble is formed at the first is called the minimum bubbling velocity, and the fluidized bed where such a phenomenon happens is called a bubbling fluidized bed.
In the meantime, in case of a high-pressure fluidized bed process carried out at high pressure, as shown in FIG. 1, if one end of the pressure probe 5 of a differential pressure gauge is mounted at the lower portion of the solid bed and the other end is exposed to the air, it is impossible to measure differential pressure because there is a great difference between the inside pressure of a fluidized bed reactor and atmospheric pressure.
FIG. 3 is a configurative diagram of a conventional fluidized bed system showing a method for measuring differential pressure using two pressure probes. As shown in FIG. 3, because the two probes 5 of a differential pressure gauge 6 are all mounted inside the fluidized bed, the probe immersed in the solid bed measures the sum of pressure of the system and head by particles existing at the upper part of the probe 5, and the probe 5 which is not immersed in the solid bed measures pressure of the system. Therefore, the conventional fluidized bed system uses the method for measuring only pressure drop by the particles due to a difference between the two measured values.
In general, one of the probes 5 of the differential pressure gauge 6, the probe of high pressure, namely, high point marked with + is mounted at the lower part of the fluidized bed, and the probe having low pressure, namely, low point marked with − is mounted at the upper part of the fluidized bed in order to measure a differential pressure formed by the solid bed.
Meanwhile, because pressure applied to the probe is increased if the height of the solid bed existing at the upper part of the pressure probe 5 for measuring the differential pressure which is illustrated in FIG. 1 is increased, the differential pressure is also increased. Moreover, as shown in FIG. 3, if the height of the solid bed existing between the (+) probe 5 and the (−) probe 5 is increased, pressure applied to the (+) probe 5 is increased but there is no change in pressure applied to the (−) probe 5 such that the differential pressure is increased. As described above, using the characteristics that the differential pressure is increased as the height of the solid bed is increased, the high-temperature and high-pressure fluidized bed system of which the inside is invisible measures the differential pressure to infer the height of the solid bed inside the fluidized bed.
In order to calculate the height (H) of the solid bed using the differential pressure measured under the bubbling fluidized bed condition, the following mathematical formula 1 is generally used.
                    H        =                                            Δ              ⁢                                                          ⁢              P                                                      (                                  1                  ⁢                                      -                                    ⁢                                      ɛ                    mf                                                  )                            ⁢                              (                                                      ρ                    s                                    ⁢                                      -                                    ⁢                                      ρ                    g                                                  )                                              ⁢                                    g              c                        g                                              Mathematical        ⁢                                  ⁢        Formula        ⁢                                  ⁢        1            
wherein H is height (m) of the solid bed under the fluidization condition, ΔP is a pressure drop (differential pressure) [Pa] by the solid bed, εmf is a voidage [−] of the solid bed under the minimum fluidization condition, ρs is density [kg/m3] of solid, ρg is density [kg/m3] of gas, gc is a gravitational acceleration constant, 1 [(kgm)/(Ns2)], and g is acceleration of gravity, 9.8 [m/s2].
As shown in the mathematical formula 1, in order to calculate the height of the solid bed using the differential pressure measured in the fluidized bed, various properties, such as the voidage of the solid bed under the minimum fluidization condition, the density of solid and the density of gas, are needed, and especially, in case that temperature and pressure are changed, the density of gas is also changed. Therefore, it is possible to calculate the height of the solid bed only when a correct value of the density of gas is given.
Furthermore, it is difficult to correctly measure the voidage of the minimum fluidization condition because the voidage of the minimum fluidization condition is varied according to the size of particles, sphericity, temperature, pressure and so on.
Additionally, because the density of solid is varied according to reaction time when the size and components of particles are varied by reaction inside the fluidized bed, it is difficult to exactly measure the height of the solid bed.
Finally, through the conventional method for measuring differential pressure in the high-temperature and high-pressure fluidized bed reactor accompanying reaction, it is difficult to exactly measure the height of the solid bed.